A water-absorbent resin (SAP/Super Absorbent Polymer) is a water-swellable and water-insoluble polymer gelling agent, and is frequently used in various fields of sanitary articles such as disposable diapers and sanitary napkins, water retention agents for agricultural and horticultural use, and water stopping materials for industrial use, and the like.
For the water-absorbent resin, many monomers and hydrophilic polymers are used as the raw materials thereof, and the most industrially produced is, from the viewpoint of water absorption performance, a polyacrylic acid (salt)-based water-absorbent resin in which acrylic acid and/or a salt thereof is used.
Due to improvement of performance of disposable diapers, which are a main application of the water-absorbent resin, the water-absorbent resin is required to have various functions (improved physical properties). Specifically, the water-absorbent resin is required to have various physical properties such as gel strength, water-soluble content, moisture content, water absorption speed, liquid permeability, particle size distribution, urine resistance, antibacterial property, damage resistance, powder fluidity, deodorization property, anti-coloring property, low dust, and low residual monomer, as well as water absorption capacity under no load and water absorption capacity under load which are basic physical properties.
Such a water-absorbent resin can be made into various forms such as sheet form, fiber form, film form, and the like, but are generally often made into powder form or particle form. Regarding a water-absorbent resin in powder form or particle form, it is known that the water absorption performance, the handleability, and the feeling in use vary depending on the particle diameter, the particle size distribution, or the like thereof. Thus, a water-absorbent resin in powder form or particle form which has an appropriately controlled particle diameter or particle size distribution is desired.
Examples of a main method for producing the water-absorbent resin in powder form or particle form include an aqueous solution polymerization method and a reverse phase suspension polymerization method. In the aqueous solution polymerization method, before or after drying, it is necessary to pulverize and classify a hydrous gel crosslinked polymer obtained in a polymerization step, so that the production process becomes complicated. Therefore, an advanced operating technique is necessary for stable operation.
Meanwhile, in the reverse phase suspension polymerization method, particle size control is possible at the stage of polymerization, and, for example, a method in which the ratio of a monomer relative to a hydrocarbon solvent is decreased and time is taken to drip the monomer (Patent Literature 1), a method in which a monomer aqueous solution is dripped to a hydrophobic organic solvent containing a specific phosphate-based dispersing agent (Patent Literature 2), a method in which polymerization is performed at multiple stages (Patent Literature 3), and a method in which upward flow of a dispersion medium solution is used by using a reaction device having a special shape (Patent Literature 4) have been proposed.